1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a band-pass filter, and more particularly, to a surface acoustic wave device including a plurality of band-pass filters.
2. Description of the Related Art
In recent years, multiband-capable portable telephones which include a plurality of communication systems have been developed. Furthermore, with the increase in the number of subscribers, portable telephones using a plurality of frequency bands such as the 800 MHz band of the Japanese PDC have also been developed.
These portable telephones are required for wide-band interstage band-pass filters in order to cover a plurality of frequency bands.
Moreover, in recent portable telephones, systems in which the transmission-side frequency band and the reception-side frequency band are close to each other increase in the number, and hence it is necessary to increase the steepness of the filter characteristics in the immediate vicinity of a desired pass band.
Japanese Unexamined Patent Application Publication No. 9-121138 discloses an example of a filter device capable of covering a plurality of pass bands. FIG. 17 is a circuit diagram for explaining the filter device according to this conventional art. Herein, the input terminal and the output terminal of filter elements 101 and 102 each including a surface acoustic wave filter are each shared between these filter elements 101 and 102. Specifically, the input ends of the filter elements 101 and 102 are connected to the input terminal 103, and the output ends of the filter elements 101 and 102 are connected to the output terminal 104. By causing the input terminal and output terminal to be shared between the filter elements 101 and 102, a reduction in the size and weight of a radio-signal portion is achieved.
Transmission lines 105 and 106 are connected between the input terminal 103 and one filter element 101, and between the output terminal 104 and the one filter element 101, respectively. Also, a capacitor 107 is connected between the filter element 102 and the input terminal 103, and an inductance element 108 is connected between the connection point between the input terminal 103 and the capacitor 107 and the ground potential. Similarly, a capacitor 109 is connected between the output end of the filter element 102 and the output terminal 104, and an inductance element 110 is connected between the connection point between the capacitor 109 and the output terminal 104 and the ground potential.
That is, phase adjusting circuits including the transmission lines 105 and 106 are connected between the filter element 101 and the input terminal 103 and the output terminal 104, respectively. Also, phase adjusting circuits including the capacitor 107 and the inductance element 108, and phase adjusting circuits constituted of the capacitor 109 and the inductance element 110 are connected between the filter element 102 and the input terminal 103 and the output terminal 104, respectively.
By providing the above-described phase adjusting circuits, each of the filter elements 101 and 102 is brought into a high impedance state in the band where it is not required, whereby superior pass band characteristics are obtained.
In addition, in the filter device according to this conventional art, a structure in which the above-described capacitors 107 and 109 and inductance elements 108 and 110 are disposed inside or outside the package of the filter device, or a structure in which the above-described transmission lines 105 and 106 are disposed on the laminated portion of the package, is disclosed.
However, as described in the above-described conventional art, when capacitors and inductance elements are provided inside or outside the package of the filter device in order to form phase adjusting circuits, the package itself inevitably becomes large, or the mounting area when the filter device is mounted on a circuit board of a portable telephone or other device must become large.
Also, when the transmission lines are disposed on the laminated portion of the package, fine-adjustments of the impedance cannot be performed if another surface acoustic wave filter is mounted on the same package. This requires that an exclusive package for each filter to be mounted must be used.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device including a plurality of surface acoustic wave filters having different center frequencies and sharing an input terminal and an output terminal therebetween, the plurality of surface acoustic wave filters being connected in parallel, which allows the number of inductance elements and capacitor elements provided inside or outside the package to be reduced, and which enables the mounting area when the device is mounted on a circuit board or other substrate to be decreased.
In accordance with a preferred embodiment of the present invention, a surface acoustic wave device includes a first surface acoustic wave filter connected between an input terminal and an output terminal, a second surface acoustic wave filter having a center frequency that is different from that of the first surface acoustic wave filter, and which is connected between the input terminal and the output terminal and in parallel with the first surface acoustic wave filter, and at least one one-port surface acoustic wave resonator connected in series with the first or second surface acoustic wave filter, on at least one side between the input terminal and at least one of the first and second surface acoustic wave filters, or between the output terminal and at least one of the first and second surface acoustic wave filters. In this surface acoustic wave device, the anti-resonance frequency of the one one-port surface acoustic wave resonator is positioned on the side of higher frequencies than those of the pass band of the surface acoustic wave filter disposed on the side where the one-port surface acoustic wave resonator is connected in series.
In accordance with a particular aspect of at least one preferred embodiment of the present invention, the center frequency of the second surface acoustic wave filter is higher than that of the first surface acoustic wave filter, and a first one-port surface acoustic wave resonator and a second one-port surface acoustic wave resonator are connected in series between the second surface acoustic wave filter and the input terminal, and between the second surface acoustic wave filter and the output terminal, respectively.
In accordance with another aspect of at least one preferred embodiment of the present invention, a plurality of the one-port surface acoustic wave resonators are connected in a multi-stage series connection.
In accordance with another particular aspect of at least one preferred embodiment of the present invention, at least two of the one-port surface acoustic wave resonators are connected in series, on at least one side between the first or second surface acoustic wave filter and the input terminal, or between the first or second surface acoustic wave filter and the output terminal, and the plurality of one-port surface acoustic wave resonators which are connected in series with each other, are arranged so as to differ, from each other, in the frequency determined by the pitch of the interdigital transducers (hereinafter referred to as a xe2x80x9cIDTsxe2x80x9d) thereof.
In accordance with another aspect of various preferred embodiments of the present invention, each of the first and second surface acoustic wave filters includes a longitudinally-coupled surface acoustic wave filter having three IDTs disposed along the surface wave propagation direction.
In accordance with another particular aspect of at least one preferred embodiment of the present invention, in the first and second longitudinally-coupled surface acoustic wave filters each having three IDTs, the first and second longitudinally-coupled surface acoustic wave filters are connected in parallel with each other so that the IDTs on both sides in the surface wave propagation direction, of the first and second longitudinally-coupled surface acoustic wave filters share the input (output) terminal therebetween, and simultaneously so that the center IDTs of the first and second longitudinally-coupled type surface acoustic wave filters share the output (input) terminal therebetween.
In accordance with another aspect of various preferred embodiments of the present invention, in at least one one-port surface acoustic wave resonator of the above-described one-port surface acoustic wave resonators, IDTs are weighting via a thinning method or other suitable process.
In accordance with still another aspect of preferred embodiments of a surface acoustic wave device in accordance with the present invention, there is further provided an inductance element additionally connected in parallel with each of the input and output terminals.
Preferably, the inductance value of the inductance element additionally connected in parallel with the input terminal is arranged so as to differ from that of the inductance element additionally connected in parallel with the output terminal.
In accordance with a further particular aspect of various preferred embodiments of the present invention, the electrode film thickness of the first surface acoustic wave filter is arranged so as to differ from that of the second surface acoustic wave filter.
In accordance with a yet further particular aspect of various preferred embodiments of the present invention, the first surface acoustic wave filter and the second surface acoustic wave filter are connected in parallel with each other on a piezoelectric substrate.
In accordance with another particular aspect of various preferred embodiments of the present invention, the input terminal and/or the output terminal are arranged to be balanced-signal terminals.
A communication device in accordance with another preferred embodiment of the present invention is characterized in that it includes a surface acoustic wave device according to other preferred embodiments of the present invention, as a band-pass filter.
In accordance with another preferred embodiment of the present invention, the input terminal and the output terminal of the first and second surface acoustic wave filters having different center frequencies, are shared between these surface acoustic wave filters, and at least one one-port surface acoustic wave resonator is connected in series with the first or second surface acoustic wave filter, on at least one side between the input terminal and at least one of the first and second surface acoustic wave filters, or between the output terminal and at least one of the first and second surface acoustic wave filters. Hence, impedance matching can be easily achieved in the construction wherein the first and second surface acoustic wave filters are connected in parallel with each other.
Since the anti-resonance frequency of the above-described one-port surface acoustic wave resonator is preferably at a higher frequency than the frequencies of the pass band of the surface acoustic wave filter disposed on the side where the one-port surface acoustic wave resonator is connected in series, the attenuation value on the higher frequency side of the pass band of the surface acoustic wave filter disposed on the side where the one-port surface acoustic wave resonator is connected in series, can be increased.
In the conventional example, it is necessary to connect numerous elements for impedance matching to the external elements, whereas, in preferred embodiments in accordance with the present invention, the number of elements to be connected to external elements is greatly reduced, which allows the mounting area to be greatly reduced.
When a one-port surface acoustic wave resonator is connected between each of the first and second surface acoustic wave filters and the input terminal, resistance against power is greatly improved.
When a plurality of one-port surface acoustic wave resonators are connected in multi-stage series connection, resistance against power is even more improved.
When at least two one-port surface acoustic wave resonators are connected in series, on at least one side between the first or second surface acoustic wave filter and the input terminal, or between the first or second surface acoustic wave filter and the output terminal, and the plurality of one-port surface acoustic wave resonators which are connected in series with each other, differ in the frequency determined by the pitch of the IDT from each other, not only is the resistance against power greatly improved, but also the steepness on the side of higher frequencies than the frequencies of the pass band is improved and ripples occurring in the pass band are greatly reduced.
When IDTs are thinned out in at least one one-port surface acoustic wave resonator of the one-port surface acoustic wave resonators, the anti-resonance approaches the resonance frequency side, and thereby a higher attenuation value can be achieved at the vicinity of the higher frequency side of the pass band.
In preferred embodiments of the present invention, when each of the first and second surface acoustic wave filters includes longitudinally-coupled surface acoustic wave filters having three IDTs disposed along the surface wave propagation direction, the miniaturization of the first and second surface acoustic wave filters can be achieved by using the longitudinally-coupled surface acoustic wave filters.
In the construction wherein, in the first and second longitudinally-coupled type surface acoustic wave filters each having three IDTs, the first and second longitudinally-coupled surface acoustic wave filters are connected in parallel with each other so that the IDTs on both sides in the surface wave propagation direction, of the first and second longitudinally-coupled surface acoustic wave filters share the input (output) terminal therebetween, and simultaneously so that the center IDTs of the first and second longitudinally-coupled type surface acoustic wave filters share the output (input) terminal therebetween, the insertion loss in the pass band is even more reduced.
When an inductance element additionally connected in parallel with each of the input and output terminals is further provided, the impedance is matched more optimally by this inductance element.
When the inductance value of the inductance element additionally connected in parallel with the input terminal differs from that of the inductance element additionally connected in parallel with the output terminal, impedance matching can be optimized on each of the sides of the input terminal and the output terminal.
The electrode film thicknesses of the first and second surface acoustic wave filters may be substantially equal, but may instead be different from each other so as to be optimized depending on characteristics of each of the surface acoustic wave filters, which allows the degree of freedom of the ultimately achievable characteristics to be increased.
In the construction wherein the first surface acoustic wave filter and the second surface acoustic wave filter are connected in parallel with each other on a piezoelectric substrate, the miniaturization of the surface acoustic wave device in accordance with various preferred embodiments of the present invention can be achieved, and the simplification of the electrode design on the substrate and on the package side can be achieved.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features, elements, characteristics and advantages of preferred embodiments of the present invention will become more apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.